1. Field of the Invention
The present invention relates to a modular hip prosthesis system wherein various custom, selected component parts of an implant can be selected during the surgical procedure and a corresponding mill or rotary reamer guide selected to prepare the femur so that the implant can be custom fitted to a patient.
2. General Background
Increasingly, surgeons want to be able to custom fit femoral prostheses to patients. Instead of having to choose a properly sized prosthesis from a group of pre-formed implants, it would be advantageous to have a basic design which can be modified with various component parts. This would eliminate the need to maintain a large inventory and would provide better fitting implants. In addition, the femur of each patient can differ slightly so that some cutting and/or reaming of the intermedullary cavity is desireable for properly preparing the femur to receive the selected prosthesis.
Custom fitted implants are particularly important in revision cases where an implant has to be removed and replaced since old cement must be removed and bone resorption occurs in many cases. Unpredictable, proximal and/or distal bone loss of deformity often occurs which must be accommodated by the replacement prosthesis.
For initial implants, basic variations in patient anatomy are often confronted by the surgeon. Variations in intramedullary canal diameter can also occur, which if not accompanied by a properly sized implant, can result in distal toggle.
In short, it is difficult, of not impossible, to predict the exact shape of a hip implant which is suitable for all patients. Since it is impractical and would be prohibitively expensive to maintain an inventory of implants for most patients, compromises must sometimes be made in supplying implants which fit reasonably well but could be improved upon.
Several hip prostheses are known which are formed of replaceable or interchangeable component parts.
U.S. Pat. No. 3,641,590 entitled xe2x80x9cAcetabular Replacement Prosthesis and Method of Assemblingxe2x80x9d issued to Michele discloses a selective individualized technique for acetabulum socket replacement per se, or in conjunction with a hip replacement prosthesis (referring to the Michele U.S. Pat. No. 3,228,393) for a total hip replacement, designed for all ages including the very young. A selective anchorage for a cup prosthesis of a size selected from the limited number of differently sized cups is made available. Anchorage of the acetabular socket replacement conforms to variations in dimensions, shapes and positions of the (medullary) canals of the acetabulum pelvis of the individual patient and includes at least two elongated and convergent or divergent fasteners.
A removable collar of low modular of elasticity material is shown in U.S. Pat. No. 4,012,796 entitled xe2x80x9cInterpositioning Collar For Prosthetic Bone Insertxe2x80x9d issued to Weisman et al. The collar is interpositioned between a collar of a metal prosthetic hip stem implanted in the intramedullary canal of the femur and the adjacent calcar or outer edge of the bone. A flange depends from the insert between the upper portion of the stem and the inner wall of the bone. The interpositioned collar is either a full elongated tapered O-shape or it is open on one side of a tapered U-shape.
U.S. Pat. No. 4,404,691 entitled xe2x80x9cModular Prosthesis Assemblyxe2x80x9d issued to Buning et al., provides a modular hip prosthesis assembly for replacement of at least part of a joint and part of a bone shaft including a mounting component provided with a connection portion and at least two joint components of similar shape but different dimensions and which can be connected alternatively to the mounting component, each of the joint components having an engagement portion and a connection part adapted for connection to the connection portion of the mounting component, the joint components each providing part of a bone shaft and part of a joint which can cooperate with an appropriate part of a natural or artificial joint.
U.S. Pat. No. 4,578,081 entitled xe2x80x9cBone Prosthesisxe2x80x9d issued to Harder et al., discloses a bone prosthesis comprising at least one joint component replacing a natural joint half, which is provided with a shank adapted to be connected to the bone, wherein a set of joint components is provided, and the shank is designed as a bone replacement member, with a connection portion provided adapted to be connected to the bone at one end and at the other end to the shank. One of the components is a hip prosthesis with a rounded head and a hollowed hip component that connects to elongated mounting components. In another hip prosthesis embodiment, a neck with a cone shape receives a suitable joint head with an inner cone.
A femoral component for hip prosthesis is shown in U.S. Pat. No. 4,608,055 issued to Morrey et al., the prosthesis disclosed in the ""055 patent includes a stem portion and a combined integral head and neck portion. The stem portion includes a proximal portion and a distal portion which are angular related with a respect to one another and with the proximal portion including a recess formed therein for receipt of a tapered portion of the head and neck component. The head and neck component includes a substantially part spherical head portion attached to a neck portion and a tapered portion angularly attached to the neck portion via a basilar neck portion with the tapered portion being adapted to be permanently inserted into the recess portion of the proximal end of the above described stem portion. The stem portion includes a plurality of recesses on the periphery and longitudinal extent thereof for receiving fiber metal pads which are provided to allow boney ingrowth therein in order to retain the femoral component permanently installed in the proximal end of the femur.
In U.S. Pat. No. 4,676,797 entitled xe2x80x9cUnit for Resection Prosthesisxe2x80x9d, a resection prosthesis assembly unit includes a head member, an end member and an intermediate member between the head and end members, of which one member is provided with a conical pin and another member is provided with a conical pin and another member is provided with a conical bore. The latter two members are provided with respective first surfaces extends transversely to an insertion direction and which face one another and are spaced apart to define a recess when the two members are connected together. One of the two members is further provided with a second surface extending in the insertion direction, and the recess being provided to receive a wedge insertable into the recess to bear against the first surfaces for forcing the two members apart while the forces exerted by the wedge are absorbed by the first surfaces and the wedge is guided by the second surface.
One type of cutting guide for use with a reamer in preparing the proximal end of the femur for a hip prosthesis is described in U.S. Pat. No. 4,777,942, entitled xe2x80x9cBone Milling Instrument.xe2x80x9d The ""942 patent discloses a milling instrument which is inserted into a medullary cavity and a spindle which is linked to the caliper at an angle. The spindle carries a milling cutter as well as a guide shoe at the distal end which slides within a guideway at the distal end of the caliper. The instrument is able to cut a circular arc corresponding to the boundary line between the spongiosa and cortical tissue in the region of the calcar arc.
The present invention provides a rotary reamer guide apparatus for preparing the femur for a hip prosthesis which can then be custom fitted to a particular patient by a surgeon prior to surgical insertion of the prosthesis.
The apparatus includes preferably a V-shaped guide body having a lower end base portion that is adapted to extend into the intermedullary canal of the femur during use, with an upper end portion comprised of a pair of spaced apart struts. The lower end of the base includes one or more receptacles for holding the end of a mill or rotary reamer.
In the preferred embodiment, the mill provides at its lower end portion a hemispherical surface and the receptacle comprises a pair of spaced apart hemispherical receptacles, for containing the hemispherical portion of the mill in one of two pre-selected cutting positions for the lower end of the reamer.
A curved, transverse guide rail is provided which attaches at a removable connection with the upper end of the guide body at one of the struts, the guide rail including a cantilevered arm having a curved surface that is adapted to engage and guide the mill during preparation of the intermedullary canal of the patient""s femur before receiving a hip prosthesis.
Because the guide rail is removable from the guide body, the present invention provides a modular kit wherein a plurality of different guide rails can be used for creating cuts of different pre-selected configurations in the intermedullary canal. Thus, the surgeon can select a particular guide rail configuration and resultant cut for the intermedullary canal depending upon the particular patient""s needs and depending upon a particular selected modular prosthesis that will be implanted.
The guide body preferably defines an envelope having a size and shape that corresponds to the shape of the upper end portion of the prosthesis body so that the guide body and the cut made by the reamer closely corresponds to the actual shape of the prosthesis to be implanted, thus eliminating the problem of a poor fit.
In the preferred embodiment, the guide rail provides a cantilevered arm that extends away from the V-shaped guide body, connecting thereto by means of a removable connection, such as, for example, a pair of spaced apart pins that downwardly extends from the cantilevered arm into sockets provided on the guide body. As an alternate construction, the connection can be in the form of a dove-tail shaped connection member carried by the guide rail, arm and slidably interfacing with a dove-tail shaped recess in the guide body. The guide rail can provide connectors on both the superior and inferior surfaces to that each guide rail can be used to make both anterior and posterior cuts as desired.